U/Pb and Pb/Pb zircon ages from granitoid rocks of Wallagga area: constraints on magmatic and tectonic evolution of Precambrian rocks of western Ethiopia

摘要

The Precambrian rocks in western Ethiopia consist of high- and low-grade terranes intruded by granitoids with a wide compositional spectrum. The formation ages of these granitoid rocks are, so far, poorly understood. Single-grain zircon Pb/Pb evaporation and conventional U/Pb dating conducted on four granitoids places time constraints on their emplacement and tectonothermal events. Three granitoid magmatic events were identified at 815 Ma, 700-730 Ma, and 620-625 Ma, which were marked by emplacement of the calc-alkaline Ujjukka granite and granodiorite, the anatectic Suqii-Wagga two-mica granite and the Guttin K-feldspar megacrystic granite, and the anorogenic Ganjii monzogranite, respectively. We interpret the 815 Ma age to mark a major magmatic episode in this pan of Africa. A tectonothermal event at approx 630 Ma preceded the emplacement of the within-plate granitoids at 620-625 Ma. The decrease of ages from the calc-alkaline to anorogenic granitoids suggests a shift of magmatic styles and tectonic setting of the granitoids over a period of 200 million years. The Suqii-Wagga and Guttin granites, representing the granitoid population in the migmatitic terrane, formed as pan of the successive evolution of the granitoid magmatism in the region. The presence of xenocrystic zircons of Mesoproterozoic ages in both granitoid populations emplaced into the low-grade volcanosedimentary sequence and the high-grade, often migmatitic, gneisses suggest contribution of pre-Pan-African crust to the origin and evolution of the granitoids. Conventional U/Pb studies of zircons from the Guttin K-feldspar megacrystic granite and the Ganjii monzogranite yielded upper intercept ages of approx 3 Ga and approx 2 Ga, respectively, possibly indicating the presence of reworked Archean-Proterozoic crustal material.